Method of forming resist pattern and thermally stable and highly resolved resist pattern

ABSTRACT

Positive photoresist compositions are provided which contain 
     (a) a mixture of mono-, di-, and triesters of 2,3,4-trihydroxybenzophenone and 1,2-naphthoquinone-2-diazo-4-sulfonic acid and 
     (b) a novolak resin selected from resins prepared from a phenolic component having a high p-cresol content or from a mixture of formaldehyde and an aromatic aldehyde.

This is a divisional of copending application Ser. No. 07/376,971 filedon 7/6/89, U.S. Pat. No. 4,943,511, which is a File Wrapper Continuationof Ser. No. 07/229,088 filed Aug. 5, 1988 abandoned, which is aContinuation-In-Part of Ser. No. 07/175,706 filed Mar. 31, 1988, nowabandoned.

CROSS REFERENCE TO OTHER APPLICATION

This application is related in subject matter to application Ser. No.175,473 filed Mar. 31, 1988, now abandoned of R. M. Lazarus, R. Kautz,and S. S. Dixit entitled "MIXED ALDEHYDE NOVOLAK RESINS AND HIGHCONTRAST HIGH THERMAL STABILITY POSITIVE PHOTORESISTS PREPAREDTHEREFROM", which application was filed concurrently with thisapplication.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates generally to positive photoresistscontaining cresol-aldehyde novolak resins having a high p-cresol contentand a photoactive component comprising mono-, di-, and triesters of2,3,4-trihydroxybenzophenone and 1,2-naphthoquinone-2-diazo-4-sulfonylchloride.

(2) Description of the Prior Art

It is well known in the art to produce positive photoresist formulationssuch as those described in U.S. Pat. Nos. 3,666,473; 4,115,128; and4,173,470. These include alkali-soluble phenol- (or cresol-)formaldehyde novolak resins together with light-sensitive materials(sometimes called "photoactive components"), usually substitutednaphthoquinonediazide compounds. The resins and sensitizers aredissolved in an organic solvent or mixture of solvents and are appliedas a thin film or coating to a substrate suitable for the particularapplication desired.

The novolak resin component of these positive photoresist formulationsis soluble in aqueous, alkaline solutions, but the naphthoquinonesensitizer acts as a dissolution rate inhibitor with respect to theresist. Upon exposure of selected areas of the coated substrate toactinic radiation, however, the sensitizer undergoes a radiation inducedstructural transformation, and the exposed areas of the photoresistcoating are rendered more soluble than the unexposed areas. Thisdifference in solubility rates causes the exposed areas of the positivephotoresist coating to be dissolved when the substrate is immersed inalkaline developing solution while the unexposed areas are largelyunaffected, thus producing a positive relief pattern on the substrate.

In most instances, the exposed and developed substrate will be subjectedto treatment by a substrate etchant solution or by dry etching. Thepositive photoresist coating protects the coated areas of the substratefrom the etchant and thus the etchant is only able to etch the uncoatedareas of the substrate, which in the case of a positive photoresist,correspond to the areas that were exposed to actinic radiation. Thus, anetched pattern can be created on the substrate which corresponds to thepattern of the mask, stencil, template, or the like that was used tocreate selective exposure patterns on the coated substrate prior todevelopment.

The relief pattern of photoresist on substrate produced by the methoddescribed above is useful for various applications including, forexample, as an exposure mask or pattern such as is employed in themanufacture of miniaturized integrated electronic components.

The properties of a photoresist composition which are important incommercial practice include the photospeed of the resist, developmentcontrast, resist resolution, and resist adhesion. Also, since thephotoresist may have to withstand an ion implantation process which maybe conducted at high temperature, the thermal stability of thephotoresist may be important.

Increased photospeed is important for a photoresist, particularly inapplications where a number of exposures are needed, for example, ingenerating multiple patterns by a repeated process, or where light ofreduced intensity is employed such as in projection exposure techniqueswhere the light is paned through a series of lenses and monochromaticfilters. Thus, increased photospeed is particularly important for aresist composition employed in processes where a number of multipleexposures must be made to produce a mask or series of circuit patternson a substrate. These optimum conditions include a constant developmenttemperature and time in a particular development mode, and a developersystem selected to provide complete development of exposed resist areaswhile minimizing the film thickness loss in the unexposed areas.

Development contrast refers to a comparison between the percentage offilm loss in the exposed area of development with the percentage of filmloss on the unexposed area. Ordinarily, development of an exposed resistcoated substrate is continued until the coating on the exposed area issubstantially completely dissolved away and, thus, development contrastcan be determined simply by measuring the percentage of the film coatingloss in the unexposed areas when the exposed coating areas are removedentirely.

Resist resolution refers to the capability of a resist system toreproduce the smallest equally spaced line pairs and intervening spacesof a mask which is utilized during exposure with a high degree of imageedge acuity in the developed exposed spaces. In many industrialapplications, particularly in the manufacture of miniaturized electroniccomponents, a photoresist is required to provide a high degree ofresolution for very small line and space widths (on the order of onemicron or less).

The ability of a resist to reproduce very small dimensions, on the orderof one micron or less, is extremely important in the reproduction oflarge scale integrated circuits on silicon chips and similar components.Circuit density on such a chip can only be increased, assumingphotolithographic techniques are used, by increasing the resolutioncapabilities of the resist.

One approach to increasing the resolution capabilities of a resist is toexpose with shorter wavelength light. For example, exposures with mid-UVradiation (about 280-365 nm) typically give on the order of 0.25 μmbetter resolution than exposures in the near UV region (about 365-459nm).

One attempt at increasing resolution is provided in U.S. Pat. No.4,596,763 to DiCarlo et al. (issued June 24, 1986) where a positivephotoresist containing a novolak resin and a 1-naphthalene sulfonicacid, 3-diazo-3,4-dihydro-4-oxo-4-benzoyl-1,2,3-benzenetriyl esterphotosensitizer is said to provide increased photospeed while retainingor improving the resist contrast in the mid-UV region.

U.S. Pat. No. 4,499,171 to Hosaka et al. (issued Feb. 12, 1985) alsodiscloses positive photoresists which are said to possess excellentsensitivity and yield of residual film thickness. These photoresistscontain a novolak resin and a photoactive component which may be amixture of tri- and diesters of 1,2-naphthoquinonediazide-4-sulfonicacid and 2,3,4-trihydroxybenzophenone. The photoactive component mayalso contain2,3,4-trihydroxybenzophenone-1,2-naphthoquinonediazide-4-sulfonic acidmonoester.

U.S. Pats. No. 4,377,631 to Toukhy et al. (issued Mar. 22, 1983) and4,529,682 to Toukhy (issued July 16, 1985) disclose cresol-formaldehydenovolak resins made from mixtures containing meta- and para-cresol, orortho-, meta-, and para-cresol. These novolak resins may be used withconventional photosensitizers to prepare fast positive photoresistcompositions, i.e., those with fast photospeed and good resolution anddevelopment contrast. These compositions are particularly sensitive toradiation in the near-ultraviolet to conventional ultraviolet range,i.e., radiation from a wavelength of about 315 to about 450 nanometers.The ratio of ortho-, meta-, and para-cresol in the novolak resins issaid to be specific. For instance, when the novolak contains only meta-and para-cresol, a maximum of 53% para-cresol may be used (resinscontaining 55% para-cresol are specifically stated to be outside thescope of the invention in Table I of U.S. Pat. No. 4,529,682).

Positive photoresists said to have improved photospeed and rate ofdevelopment are also disclosed in U.S. Pat. No. 4,650,745 to Eilbeck(issued Mar. 17, 1987). The resist contains a novolak resin preparedfrom, e.g., a 45% meta-cresol, 55% para-cresol mixture, anaphthoquinonediazide sensitizer (e.g., esters of2,3,4-trihydroxybenzophenone and naphthaquinone-(1,2)-diazide-5-sulfonylchloride), a dye which absorbs light at a maximum wavelength of fromabout 330 to about 460 nanometers, and a trihydroxybenzophenonecompound.

U.S. Pat. No. 4,731,319 to Kohara et al. (issued Mar. 15, 1988)discloses positive photoresist compositions containing a naphthoquinonediazide sulfonic acid ester photosensitizer and a mixture of cresolnovolak resins containing (1) a first cresol novolak resin having aweight-average molecular weight of at least 5000 and produced from anisomeric mixture consisting essentially of 60 to 80% of m-cresol and 40to 20% of p-cresol, and (2) a second cresol novolak resin having aweight-average molecular weight not exceeding 5000 and produced from anisomeric mixture consisting essentially of 10 to 40% of m-cresol and 90to 60% of p-cresol. The proportions of the two cresol novolak resins arechosen so that the overall cresol component is comprised of from 30 to46.5% m-cresol and 70 to 53.5% p-cresol. The only photosensitizersdisclosed are 2,3,4-trihydroxybenzophenonenaphthaquinone-1,2-diazido-5-sulfonic acid ester;2,3,4,4'-tetrahydroxybenzophenone naphthoquinone-1,2-diazido-5-sulfonicacid ester; propyl gallate naphthoquinone-1,2-diazido-5 -sulfonic acidand isoamyl gallate naphthoquinone-1,2-diazido-5-sulfonic acid.

European Patent Application Publication No. 0211667 of Hosaka et al.(published Feb. 25, 1987) discloses that monooxymonocarboxylic acidesters, e.g., alkyl 3-alkoxypropionates, are solvents for positivephotoresist compositions which impart high storage stability to theresist, i.e., only a small amount of fine particles are formed duringstorage . The resist may contain novolak resins made from o-, m-, orp-cresol or mixtures thereof (e.g., a mixture of 60% m-cresol and 40%p-cresol) and 1,2-naphthoquinonediazide ester photosensitizers. Thesesensitizers include2,3,4-trihydroxybenzophenone-1,2-naphthoquinonediazide-4-sulfonic acidester, though the 5-sulfonic acid ester isomers are preferred.

European Patent Application Publication No. 0225464 of Collini et al.(published June 16, 1987) discloses composite, two-layer resiststructures. The bottom layer is a positive photoresist containing anovolak resin and a diazoquinone compound, e.g.,4',2',3'-dihydroxybenzophenone mixed esters of1-oxo-2-diazo-naphthalene-5-sulfonic acid [sic]. These photoresists aresaid to be particularly suitable in the mid ultraviolet range (300-350nanometers) and the near ultraviolet rane (365 to 450 nanometers).

E. Gipstein et al., "Evaluation of Pure Novolak Cresol-FormaldehydeResins for Deep U.V. Lithography," J. Eleotrochem. Soc.: Solid-StateScience and Technology, Vol. 129, No. 1, pp. 201-205 discusses theeffects of novolak resins made from pure o-, m-, or p-cresol on positivephotoresists which are exposed to radiation at 254 nm (in the so-called"deep U.V." region). The conventional novolak resins are said to beessentially transparent at 405 nm (i.e., the wavelength of mercuryemissions), but have a strong absorption at 254 nm. Thus, the combinedabsorption of the photoactive component and resin in the resist rendersfilms more than 1 μm thick virtually opaque at 254 nm. Gipstein et al.disclose that novolak resins made from pure o-, m- , or p-cresol aremore transparent at 254 nm than the conventional resins, the purep-cresol novolak being the most transparent.

In a reprint from the Proceedings of SPIE--The International Society forOptical Engineering (Vol. 771, March 1987) entitled "DYNALITH® Resistsfor Mid-UV Applications: Formulation Optimization for GaAs RelatedProcessing", R. M. Lazarus and S. S. Dixit discuss the properties ofDYNATITH®X-1605 and X-1608 positive photoresists. DYNATITH® X-1608 issaid to employ a novolak of greater para incorporation than is typicaland a sensitizer derived from the following diazo compound: ##STR1## TheX-1608 resist has high photospeed and high resolution properties.

Thus, there has long been a need for positive resist compositions whichexhibit high photospeeds and good resolution properties when exposed toradiation of relatively short wavelength, e.g., about 248-365 nm. It hasnow been discovered that positive photoresists comprising novolak resinshaving a high p-cresol content and a photosensitizer comprising amixture of mono-, di-, and triesters of 2,3,4-trihydroxybenzophenone and1,2-naphthoquinone-2-diazo-4-sulfonyl halide provide excellentphotospeed and resolution when exposed to radiation in the 248-365 nmrange.

SUMMARY OF THE INVENTION

In accordance with the present invention, there are provided positivephotosensitive compositions (also called positive photoresists)comprising an admixture of a 1,2-naphthoquinonediazide photosensitizerand a bindingly effective amount of a novolak resin wherein:

A. said photosensitizer comprises an admixture of

2,3,4-trihydroxybenzophenone-1,2-naphthoquinone-2-diazo-4-sulfonic acidtriester;

2,3,4-trihydroxybenzophenone-1,2-naphthoquinone-2-diazo-4-sulfonic aciddiester and

2,3,4-trihydroxybenzophenone-1,2-naphthoquinone-2-diazo-4-sulfonic acidmonoester in the following relative amounts:

triester: 50 weight percent (based on the total weight of triester,diester, and monoester) to about 75 weight percent

monoester and diester: balance, in a monoester/diester weight ratio ofabout 1/2, said wt % being based on the total weight of the mono-, di-and triester; and

B. said novolak resin comprises a resin selected from the groupconsisting of:

1. a resin prepared by condensing an aliphatic aldehyde, aromaticaldehyde or mixture thereof with a phenolic component comprising amixture of p-cresol and m-cresol containing at least about 60 weightpercent p-cresol, based on the total weight of p-cresol and m-cresol,and a sufficient amount of m-cresol to render said novolak resin solublein an aqueous, alkaline solution, said phenolic component beingessentially free of o-cresol; and

2. a novolak resin which is the condensation product of (a) a phenol,phenol derivative or mixture thereof and (b) a mixture of formaldehydeor a formaldehyde precursor and an aromatic aldehyde.

Also provided in accordance with the present invention are positivephotosensitive compositions containing components A and B describedabove and a solvent, wherein said solvent is selected from the groupconsisting of (1) a mixture of ethyl Cellosolve acetate, butyl acetate,and xylene; (2) a mixture of propylene glycol monomethyl ether acetate,butyl acetate, and xylene; (3) ethyl-3-ethoxy propionate; (4) propyleneglycol monomethyl ether acetate; and (5) a mixture of propylene glycolmonomethyl ether acetate, butyl acetate, and ethyl-3-ethoxy propionate.

Further provided in accordance with the present invention is a method offorming a resist pattern on a substrate, said process comprising:

A. coating said substrate with a layer of a positive photoresistcomposition;

B. exposing said layer patternwise to actinic radiation; and

C. removing the exposed portion of said layer with an aqueous alkalinedeveloper for the exposed resist composition to uncover the areas of thesubstrate beneath the exposed portions; said positive photoresistcomprising, prior to exposure, an admixture of a1,2-naphthoquinonediazide photosensitizer and a bindingly effectiveamount of a novolak resin wherein:

I. said photosensitizer comprises an admixture of

2,3,4-trihydroxybenzophenone-1,2-naphthoquinone-2-diazo-4-sulfonic acidtriester;

2,3,4-trihydroxybenzophenone-1,2-naphthoquinone-2-diazo-4-sulfonic aciddiester; and

2,3,4-trihydroxybenzophenone-1,2-naphthoquinone-2-diazo-4-sulfonic acidmonoester in the following relative amounts:

triester--about 50 wt % to about 75 wt %,

monoester and diester--balance, in a mono-ester/diester weight ratio ofabout 1/2, said wt % being based on the total weight of the mono-, di-,and triester; and

II. said novolak resin comprises a resin selected from the groupconsisting of:

a. a resin prepared by condensing an aliphatic aldehyde, aromaticaldehyde or mixture thereof with a phenolic component comprising amixture of p-cresol and m-cresol containing at least about 60 weightpercent p-cresol, based on the total weight of p-cresol and m-cresol,and a sufficient amount of m-cresol to render said novolak resin solublein an aqueous, alkaline solution, said phenolic component beingessentially free of o-cresol; and

b. a novolak resin which is the condensation product of (a) a phenol,phenol derivative, or mixture thereof and (b) a mixture of formaldehydeor a formaldehyde precursor and an aromatic aldehyde.

Also in accordance with the present invention there is provided asubstrate coated with a thermally stable and highly resolved, exposedresist pattern, said resist pattern being formed from a positivephotoresist composition which, prior to exposure to actinic radiation,comprises

A. a photosensitizer comprising an admixture of

2,3,4-trihydroxybenzophenone-1,2-naphthoquinone-2-diazo-4-sulfonic acidtriester;

2,3-4-trihydroxybenzophenone-1,2-naphthoquinone-2-diazo-4-sulfonic aciddiester, and

2,3,4-trihydroxybenzophenone-1,2-naphthoquinone-2-diazo-4-sulfonic acidmonoester in the following relative amounts:

triester--about 50 wt % to about 75 wt %

monoester and diester--balance, in a monoester/diester weight ratio ofabout 1/2,said wt % being based on the total weight of the mono-, di- ,and triester; and

B. a novolak resin comprising a resin selected from the group consistingof:

1. a resin prepared by condensing an aliphatic aldehyde, aromaticaldehyde or mixture thereof with a phenolic component comprising amixture of p-cresol and m-cresol containing at least about 60 weightpercent p-cresol, based on the total weight of p-cresol and m-cresol,and a sufficient amount of m-cresol to render said novolak resin solublein an aqueous, alkaline solution, said phenolic component beingessentially free of o-cresol; and

2. a novolak resin which is the condensation product of (a) a phenol,phenol derivative or mixture thereof and (b) a mixture of formaldehydeor a formaldehyde precursor and an aromatic aldehyde.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The positive photoresists of the present invention comprise aphotosensitizer (also known as a "photoactive component" or "PAC") and anovolak resin binder. The photosensitizer is a mixture of mono-, di-,and triesters of 2,3,4-trihydroxybenzophenone and1,2-naphthoquinone-2-diazo-4-sulfonyl chloride. The reactants which formthe mono-, di-, and triesters have the following structures: ##STR2##and the resulting mono-, di-, and triesters have the followingstructures: ##STR3##

The above mono-, di-, and triesters are known in the art and may beprepared by known methods, such as the general procedure described inU.S. Pat. 4,499,171 to Hosaka et al., issued Feb. 12, 1985, which ishereby incorporated by reference herein.

The novolak resins useful in the practice of the present invention fallinto two categories. The first, termed herein the "high p-cresol novolakresins" are prepared by condensing an aliphatic aldehyde, aromaticaldehyde, or mixtures thereof with a phenolic component which is amixture of p-cresol and m-cresol which is essentially free of o-cresol.While it is preferable that the phenolic component contain no o-cresolwhatsoever, o-cresol is commonly present as an impurity in p-and/orm-cresol. Thus, as used herein, the phrase "essentially free ofo-cresol" means less than about 5% by weight of the phenolic componentis o-cresol.

It is desirable to maximize the p-cresol content of the phenoliccomponent in the high p-cresol resin in order to provide maximumtransparency of the positive resist in the 248-365 nm range. However,novolak resins prepared from a phenolic component which is 100% p-cresolare difficult to dissolve in the aqueous, alkaline solutions, e.g.,aqueous solutions of tetramethylammonium hydroxide, typically used asdevelopers for positive resists. Thus, the phenolic component of thenovolak resins useful in the present invention contain sufficientm-cresol to allow the resin to dissolve in an aqueous, alkaline(typically at a pH of about 12 or higher) solution. Typically, anunexposed positive photoresist containing said resin is dissolved byimmersing the unexposed resist in an aqueous solution containing about1.5% to about 2.4% tetramethylammonium hydroxide (TMAH) at about 19° C.for about 60 seconds. Alternatively, the TMAH solution can be applied tothe unexposed resist by the so-called "spray puddle" process for about20-42 seconds. In general, this dissolution is achieved by using aphenolic component which has a weight ratio of p-cresol: m-cresol offrom about 60:40 to about 80:20.

The aldehyde which may be used to prepare the high p-cresol novolakresin are aliphatic aldehydes, such as formaldehyde, aromatic aldehydessuch as 4-hydroxybenzaldehyde, and mixtures of aliphatic and aromaticaldehydes, such as formaldehyde and 4-hydroxybenzaldehyde.

In preparing the high p-cresol novolak resins of the present invention,the ratio of aldehyde component to phenolic component should be asfollows: ##EQU1##

The second type of novolak resin useful in the present invention isprepared using a mixture of formaldehyde or a formaldehyde precursor anda monohydroxy aromatic aldehyde. These resins are termed herein the"mixed aldehyde novolak resins" and are the subject of a co-pendingapplication Ser. No. 175,473 filed Mar. 31, 1988, now abandoned of R. M.Lazarus, R. Kautz, and S. S. Dixit, which application is herebyincorporated by reference herein.

The mixed aldehyde novolak resins useful in the present invention areprepared from (1) phenol or a phenol derivative, and (2) a mixture ofaldehydes which contains formaldehyde (or a formaldehyde precursor) andan aromatic aldehyde. The phenol and phenol derivatives useful inpreparing these mixed aldehyde novolak resins include, but are notlimited to, phenol, m-cresol, p-cresol, o-cresol, and mixtures thereof.Other examples of phenol derivatives may be found in H. Hiraoka,"Functionally Substituted Novolak Resins: Lithographic Applications,Radiation Chemistry, and Photooxidation", Materials ForMicrolithography, ACS Symposium Series No. 266 (1984) which is herebyincorporated by reference herein. They may also contain othersubstituents on the aromatic ring including, for example, alkyl orhalogen moieties.

The mixed aldehydes useful in preparing the mixed aldehyde novolakresins useful in the present invention contain formaldehyde or aformaldehyde precursor. As used herein, "formaldehyde precursor" refersto compounds, such as 1,3,5-S-trioxane and paraformaldehyde which, underthe reaction conditions employed to prepare the novolak resin willproduce formaldehyde. As used herein, the term "formaldehyde" isunderstood to include both formaldehyde per se and formaldehydeprecursors. The second component of the aldehyde mixture is any aromaticaldehyde which will form a novolak resin with phenol or a phenolderivative. In general, these aromatic aldehydes include, but are notlimited to, compounds having the general formula: ##STR4## where R ishalogen (e.g. chlorine), alkyl (e.g. C₁ -C₄ alkyl). Examples ofpreferred aromatic aldehydes include benzaldehyde, 2-chlorobenzaldehyde,and monohydroxy aromatic aldehydes such as 2-hydroxybenzaldehyde,3-hydroxybenzaldehyde, and 4-hydroxybenzaldehyde. Mixtures of aromaticaldehydes may also be used. The monohydroxy benzaldehydes areparticularly preferred since they produce novolak resin which may beused to make high resolution, high thermal stability positivephotoresists.

The relative amounts of the phenol or phenol derivative and mixedaldehydes used to prepare the mixed aldehyde novolak resins used in thepresent invention may vary considerably. In general, these relativeamounts are chosen to produce novolak resins having high melting points(Tg), high degree of substitution at the bridging carbons and relativelylow molecular weights (compared to novolak resins prepared fromformaldehyde only). Typically, the following amounts of reactants areemployed to produce the mixed aldehyde novolak resins of this invention.##EQU2## In a preferred embodiment, the phenol component is a mixture ofphenol derivatives; namely, a mixture of m-cresol and p-cresol. Whensuch a mixture is employed, it typically contains the following relativeamounts of m- and p-cresol; ##EQU3##

Given the above reactants and their relative amounts, one of ordinaryskill in the art would be capable of preparing the high p-cresol andmixed aldehyde novolak resins of the present invention without undueexperimentation. Basically, all of the ingredients are placed in asuitable reaction vessel and a nonreactive solvent added. An acidcatalyst, such as p-toluenesulfonic acid, is added in a mole ratio ofmoles catalyst/moles total cresol of about 0.01 to about 0.04. Thereaction mixture is then raised to reflux temperature and refluxmaintained until no more by-product water is produced, indicating thatthe reaction is complete.

The positive photoresists of the present invention exhibit highsensitivity to actinic wavelengths between about 248 and about 365 nmwhile at the same time achieving excellent resolution. It is believedthere are three critical elements in the positive resists of thisinvention which combine to achieve this high sensitivity and resolution.Those elements are (1) the relatively high p-cresol content in the highp-cresol novolak resins, when they are used, (2) the chemical structureof the photosensitizer and (3) the relative amounts of the mono-, di-,and triester in the photosensitizer. Thus, for example, positive resistsusing photosensitizers which are the esters of2,3,4-trihydroxybenzophenone and 1,2-naphthoquinone-2-diazo-5-sulfonylchloride are not nearly as sensitive to actinic radiation in the 248-365nm range as are the resists of the present invention which employ theesters of 2,3,4-trihydroxybenzophenone and 1,2-naphthoquinone-2-diazo4-sulfonyl chloride. Likewise, positive resists formulated with themono-, di-, and triesters of 2,3,4-trihydroxybenzophenone and1,2-naphthoquinone-2-diazo-4-sulfonyl chloride, but with relativeamounts of the mono-, di-, and triester substantially outside the rangeof the present invention do not perform as well as those of the presentinvention, those having a higher triester content not being as sensitivein the 248-365 nm range as are the resists of the present invention, andthose with a lower triester content having too high a photospeed, poorthermal stability, and high film loss.

The fact that the positive photoresists of the present invention exhibithigh sensitivity to actinic wavelengths between about 248 nm and about365 nm means that these resists are especially advantageous when used inprocesses in which the actinic radiation is produced by a laser, e.g.,radiation in the 248 nm range.

The critical components of the positive photoresist formulation; namely,the novolak resins of the present invention and the photosensitizercomponent, can be employed over a wide range of relative amounts. Ingeneral, it is necessary only that sufficient photosensitizer beemployed to produce a resist which is photoimagable and that the amountof novolak resin be bindingly effective. In terms of relative amounts,the positive photoresists of this invention may contain from about 2parts by weight (pbw) novolak resin per pbw photosensitizer component toabout 6 pbw novolak resin per pbw photosensitizer component. Preferably,a ratio of about 4 pbw novolak resin per pbw photoactive component isemployed.

The positive photoresists of the present invention contain, in additionto the novolak resin and photosensitizer, a solvent. Examples ofsolvents which may be employed include, but are not limited to,propylene glycol monomethyl ether acetate (PMA), ethyl Cellosolveacetate (ECA), ethyl-3-ethoxy propionate (EEP), mixtures of ECA, butylacetate and xylene, mixtures of PMA, butyl acetate and xylene, andmixtures of PMA, butylacetate, and EEP. PMA and EEP are preferred overECA since they exhibit lower toxicity. EEP is particularly preferred inthat it provides a more stable environment for the components, andprovides better particulate stability, i.e., the tendency to formparticles in the resist upon storage is reduced. The use of EEP alonealso produces striation-free coatings which have a more reproduciblephotospeed than, for example, the solvents which are three componentmixtures.

The amount of solvent in the resist may vary considerably. Generally,all that is required is that sufficient solvent be employed to produce adesired coating thickness under the given coating conditions, e.g., spinspeed and pre-bake temperature. Typically, however, enough solvent isadded to the positive photoresist formulation to produce a photoresisthaving about 25-30% solids.

When mixtures of solvents are employed, they are typically used in thefollowing relative amounts:

    ______________________________________                                        Solvent      Approx. Amount (wt)                                              ______________________________________                                        ECA          80%                                                              Butyl acetate                                                                              10%                                                              Xylene       10%                                                              PMA          80%                                                              Butyl acetate                                                                              10%                                                              Xylene       10%                                                              PMA          65%                                                              Butyl acetate                                                                              30%                                                              EEP           5%                                                              ______________________________________                                    

The positive resists of this invention may also contain variousadditives such as colorants, striation aids, and the like. Also, thenovolak resin component of the resist may be comprised entirely of thehigh p-cresol or mixed aldehyde resin or may contain up to about 35weight percent (based on the total weight of the resin component of theresist) of a conventional novolak resin, such as a cresol/formaldehydenovolak resin prepared using a mixture of 65-70% m-cresol and 30-35%p-cresol.

In fact, when the high p-cresol resins are employed in the photoresistformulation, it is preferred that the novolak resin component of theresist contain about 65 weight percent high p-cresol novolak resin andabout 35 weight percent conventional novolak resin in order to minimizefilm loss and maximize thermal stability.

The following examples illustrate the present invention and itsadvantages.

EXAMPLE 1

This example illustrates a typical procedure for making the novolakresins of the present invention.

The following reagents were used in the amounts indicated below toprepare a high p-cresol novolak resin:

    ______________________________________                                        Reagent          Amount                                                       ______________________________________                                        m-cresol         325 g.                                                       p-cresol         603 g.                                                       formaldehyde (37%)                                                                             443 g.                                                       oxalic acid       71 g.                                                       ______________________________________                                    

All reagents were combined and heated at reflux. After an appropriateheating period, water was stripped off and the resulting resin wasdissolved in methanol. The resin was then purified by precipitation withwater.

EXAMPLES 2-19

Other high p-cresol novolak resins prepared in a manner similar to thatof Example 1 using the reagents indicated in Table I below:

                  TABLE I                                                         ______________________________________                                        Ex. No.  Form..sup.1  p-cresol.sup.2                                                                         m-cresol.sup.3                                 ______________________________________                                         2       77           70       30                                              3       77           80       20                                              4       66           65       35                                              5       66           70       30                                              6       66           75       25                                              7       64           60       40                                              8       64           65       35                                              9       64           65       35                                             10       64           70       30                                             11       64           75       25                                             12       62           65       35                                             13       62           70       30                                             14       62           75       25                                             15       60           60       40                                             16       60           65       35                                             17       60           70       30                                             18       60           75       25                                             19       60           80       20                                             ______________________________________                                         .sup.1 Moles formaldehyde/100 moles cresols                                   .sup.2 Approx. moles mcresol/100 moles cresols                                .sup.3 Approx. moles pcresol/100 moles cresols                           

EXAMPLE 20

A mixed aldehyde novolak resin was prepared in a manner similar to thatof Example 1 using the following reagents in the following amounts:

m-cresol

p-cresol

trioxane

2-hydroxybenzaldehyde

p-toluene sulfonic acid catalyst ##EQU4##

EXAMPLE 21

A positive photoresist in accordance with the present invention was madeby blending the following ingredients:

    ______________________________________                                        Ingredient         Wt %    Wt % (soIids basis)                                ______________________________________                                        High p-cresol novolak                                                                            32.13   13.87                                              resin from Example 1                                                          Novolak resin made from phenolic                                                                 18.40   7.36                                               component containing 65 wt %                                                  m-cresol and 35 wt % p-cresol                                                 Photosensitizer    5.30    5.30                                               Alizarin Yellow A dye                                                                            0.30    0.30                                               EEP solvent        43.53   74.02                                              Striation control additive                                                                       0.19    0.01                                                                  100.00  100.86                                             ______________________________________                                    

The photosensitizer used was a mixture of the following compounds in therelative amounts indicated:

    ______________________________________                                        2,3,4-trihydroxybenzophenone-1,2-naphthoquinone-                                                          10.56%                                            2-diazo-4-sulfonic acid monoester ("monoester")                               2,3,4-trihydroxybenzophenone-1,2-naphthoquinone-                                                          21.08%                                            2-diazo-4-sulfonic acid diester ("diester")                                   2,3,4-trihydroxybenzophenone-1,2-naphthoquinone-                                                          68.36%                                            2-diazo-4-sulfonic acid triester ("triester")                                 ______________________________________                                    

COMPARATIVE EXAMPLES A AND B

For comparative purposes two positive photoresists were formulated usingthe same ingredients as in Example 21, except that the photosensitizersemployed were as follows:

    ______________________________________                                        Comparative Resist A                                                                             Comparative Resist B                                       ______________________________________                                        Monoester   7.38 wt %  Monoester   3.11 wt %                                  Diester    13.33 wt %  Diester     7.30 wt %                                  Triester   79.29 wt %  Triester   89.59 wt %                                  ______________________________________                                    

EXAMPLE 22

This example illustrates the performance of the high p-cresolresin-containing positive photoresists of the present invention,Comparative Resists A and B and two commercially available positiveresists.

Each, in turn, of the resist compositions indicated in Tables I and IIbelow was spin coated on a track coater manufactured by Silicon ValleyGroup, California, onto thermally grown silicon/silicon dioxide coatedwafers of 4 inch diameter and 5000 Angstrom oxide thickness. A uniformcoating of 1.0 μm of resist film was obtained at spinning velocity of3.500 revolutions per minute. The coated wafers were then soft bakedeither in an air circulating oven at 95° C. for 30 minutes or on a trackwith hot plate at 95° C. for 60 seconds. The resist film was thenmeasured with a Nanospec AFT thickness measuring tool.

The coated wafers of Table I were exposed with a Cobilt CA800® fittedwith a filter to provide actinic radiation of 365 nm wavelength. AMicromaske® mask with line and space widths of varying sizes, includingsome as small as 0.75 μm, was used to provide a selective exposurepattern. Exposure times were varied in order to determine thephotospeed, i.e., the minimum amount of exposure energy (intensity×time)in millijoules/cm² which will solubilize the exposed areas of the resistso that the resist in the exposed areas will be completelyremoved/cleared during development.

The exposed resist-coated wafers produced as described above were placedin Teflon wafer boats and immersed for 60 seconds in a one-gallon Tefloncontainer containing DYNATITH® NMD-3 developer, which is an aqueousalkaline tetramethylammonium hydroxide solution stabilized at about19°±1° C. Upon removal from the developer, the wafers were rinsed indeionized water and dried by a burst of nitrogen gas or be placing themin a spin drier.

The developed wafers were then post-baked in an air circulating oven atabout 150° C. for 30 minutes to increase the adhesion and chemicalresistance of the undissolved portions of the coatings.

The coated wafers of Table II were exposed and developed in a similarmanner using a Karl Suss MA 56 aligner which provided actinic radiationat 313 nm wavelength.

                  TABLE I                                                         ______________________________________                                                                  Photo-                                                           Actinic Rad. speed    Unexposed                                  Resist       Wavelength (nm)                                                                            (mj/cm.sup.2)                                                                          film loss                                  ______________________________________                                        Resist from  365          44       0.10                                       Example 21                                                                    Comparative Resist A                                                                       365          96       1.30                                       Comparative Resist B                                                                       365          55       0.60                                       DYNALITH ®                                                                             365          80       0                                          EPR-5000 resist.sup.a                                                         AZ2400.sup.b 365          >100.sup.c                                                                             0                                          ______________________________________                                         .sup.a DYNALITH EPR5000 photoresist, sold by the Dynachem Division of         Morton Thiokol, Inc., uses a PAC based on 2,3,4trihydroxybenzophenone         esters of 1,2naphtho-quinonediazide-5 sulfonic acid.                          .sup.b AZ2400 positive photoresist sold by Shipley Co.                        .sup.c Developed for 3 minutes                                           

                  TABLE II                                                        ______________________________________                                                                  Photo-                                                           Actinic Rad. speed    Unexposed                                  Resist       Wavelenth (nm)                                                                             (mj/cm.sup.2)                                                                          film loss                                  ______________________________________                                        Resist from  313          60       0                                          Example 21                                                                    Comparative Resist A                                                                       313          140      0                                          Comparative Resist B                                                                       313          180      0                                          DYNALITH ®                                                                             313          160      0                                          EPR-5000 resist                                                               AZ2400       313          >240.sup.d                                                                             0                                          ______________________________________                                         .sup.d Three minute development                                          

The above data clearly demonstrates the superior photospeed of thepositive photoresists of the present invention.

EXAMPLE 23

This example illustrates the enhanced thermal performance of thepositive photoresists of the present invention which contain mixedaldehyde novolak resins. A positive photoresist prepared by blending thefollowing ingredients:

    ______________________________________                                                             Wt.                                                      Ingredient           in grams  Dry wt %                                       ______________________________________                                        Mixed aldehyde novolak                                                                             37.16     13.92                                          resin of Example 20                                                           Novolak resin made from                                                                            18.19     7.76                                           phenolic component containing                                                 70 wt % m-cresol and 30 wt % p-cresol                                         Photosensitizer from Example 21                                                                    5.39      5.39                                           PMA solvent          38.90     72.65                                          Alizarin Yellow A dye                                                                              0.27      0.27                                           Striation control additive                                                                         0.09      0.001                                                               100.00    99.99                                          ______________________________________                                    

The resulting resist had 27-34% solids and a viscosity of about 16-18cps.

The resist was tested in a manner similar to that described in Example22, except that an Ultratech Stepper and broad band light source wereused. For comparison, the high p-cresol resin-containing photoresist ofExample 21 was also tested, and the results are indicated below:

    ______________________________________                                                         Photospeed                                                                              Post-bake                                                 Developer (mj/cm.sup.2)                                                                           Thermal Stability                                  ______________________________________                                        Photoresist                                                                            EPD-85.sup.a,                                                                             130       130° C./30 min.                         from Ex. 21                                                                            42 seconds                                                                    spray at 19° C.                                               Photoresist                                                                            EPD-6O.sup.b                                                                              120       180° C./30 min.                         from Ex. 23                                                                            22 seoonds                                                                    spray at 19° C.                                               ______________________________________                                         .sup.a DYNALITH ®aqueous, alkaline developer (85% strength) sold by       the Dynachem Division of Morton Thiokol, Inc.                                 .sup.b DYNALITH ®aqueous, alkaline developer (60% strength) sold by       the Dynachem Division of Morton Thiokol, Inc.                            

These results indicate that the mixed aldehyde resin-containing resistcompares favorably with the high p-cresol resin-containing resist, and,in fact, develops faster (in a weaker developer) and has greaterpost-bake thermal stability than the high p-cresol resin-containingphotoresist.

We claim:
 1. A method of forming a resist pattern on a substratecomprising:1. coating said substrate with a layer of a positivephotoresist composition;
 2. exposing said layer patternwise to actinicradiation; and
 3. removing the exposed portion of said layer with anaqueous alkaline developer for the exposed resist composition to uncoverthe areas of the substrate beneath the exposed portions; said positivephotoresist comprising, prior to exposure, an admixture of a1,2-naphthoquinonediazide photosensitizer and a bindingly effectiveamount of a novolak resin wherein:A. said photosensitizer comprises anadmixtureof2,3,4-trihydroxybenzophenone-1,2-naphthoquinone-2-diazo-4-sulfonicacid triester;2,3,4-trihydroxybenzophenone-1,2-naphthoquinone-2-diazo-4-sulfonic aciddiester, and2,3,4-trihydroxybenzophenone-1,2-naphthoquinone-2-diazo-4-sulfonic acidmonoester in the following relative amounts:triester--about 50 wt % toabout 75 wt % monoester and diester--balance, in a monester/diesterweight ratio of about 1/2, said wt % being based on the total weight ofthe mono-, di-, and triester; and B. said novolak resin comprises aresin selected from the group consisting of:
 1. a resin prepared bycondensing an aliphatic aldehyde, aromatic aldehyde or mixture thereofwith a phenolic component comprising a mixture of p-cresol and m-cresolcontaining at least about 60 weight percent p-cresol, based on the totalweight of p-cresol and m-cresol, and a sufficient amount of m-cresol torender said novolak resin soluble in a aqueous, alkaline solution, saidphenolic component being essentially free of o-cresol; and2. a novolakresin which is the condensation product of (a) a mixture of m-cresol andp-cresol and (b) a mixture of formaldehyde or a formaldehyde precursorand a monohydroxy aromatic aldehyde selected from the group consistingof2-hydroxybenzaldehyde, 3-hydroxybenzaldehyde and4-hydroxybenzaldehyde.
 2. The method of claim 1 wherein the positivephotoresist further comprises a solvent, wherein said solvent isselected from the group consisting of (1) a mixture of ethyl Cellosolveacetate, butyl acetate, and xylene; (2) a mixture of propylene glycolmonomethyl ether acetate, butyl acetate, and xylene; (3) ethyl-3-ethoxypropionate; (4) propylene glycol monomethyl ether acetate, butylacetate, and ethyl-3-ethoxy propionate.
 3. The method of claim 2 whereinthe solvent is ethyl-3-ethoxy propionate.
 4. The method of claim 1wherein the novolak resin is resin B1.
 5. The method of claim 4 whereinthe mixture of p-cresol and m-cresol used to prepare novolak resin B1has a p-cresol to m-cresol weight ratio of from about 60:40 to about80:20.
 6. The method of claim 1 wherein the novolak resin is resin B2.7. The method of claim 1 wherein the positive photoresist comprises fromabout 2 parts by weight of novolak resin per part by weight ofphotosensitizer to about 6 parts by weight of novolak resin per part byweight of photosensitizer.
 8. The method of claim 7 wherein the positivephotoresist comprises about 4 parts by weight of novolak resin per partby weight of photosensitizer.
 9. The method of claim 1 wherein theactinic radiation is produced by a laser.
 10. A substrate coated with athermally stable and highly resolved, exposed resist pattern, saidresist pattern being formed from a positive photoresist compositionwhich, prior to exposure to actinic radiation, comprises:A. aphotosensitizer comprising an admixtureof2,3,4-trihydroxybenzophenone-1,2-naphthoquinone-2-diazo-4-sulfonicacid triester;2,3,4-trihydroxybenzophenone-1,2-naphthoquinone-2-diazo-4-sulfonic aciddiester, and2,3,4-trihydroxybenzophenone-1,2-naphthoquinone-2-diazo-4-sulfonic acidmoester in the following relative amounts:triester--about 50 wt % toabout 75 wt % moester and diester--balance, in a moester/diester weightratio of about 1/2, said weight being based on the total weight of themono-, di-, and triester; and B. a novolak resin comprising a resinselected from the group consisting of:1. a resin prepared by condensingan aliphatic aldehyde, aromatic aldehyde or mixture thereof with aphenolic component comprising a mixture of p-cresol and m-cresolcontaining at least about 60 weight percent p-cresol, based on the totalweight of p-cresol and m-cresol, and a sufficient amount of m-cresol torender said novolak resin soluble in an aqueous, alkaline solution, saidphenolic component being essentially free of o-cresol; and
 2. a novolakresin which is the condensation product of (a) a mixture of m-cresol andp-cresol and (b) a mixture of formaldehyde or a formaldehyde precursorand a monohydroxy aromatic aldehyde selected from the group consistingof 2-hydroxybenzaldehyde, 3-hydroxybenzaldehyde and4-hydroxybenzaldehyde.
 11. The substrate of claim 10 wherein thepositive photoresist further comprises a solvent, wherein said solventis selected from the group consisting of (1) a mixture of ethylCellosolve acetate, butyl acetate, and xylene; (2) a mixture ofpropylene glycol monomethyl ether acetate, butyl acetate, and xylene;(3) ethyl-3-ethoxy propionate; (4) propylene glycol monomethyl etheracetate; and (5) a mixture of propylene glycol monomethyl ether acetate,butyl acetate, and ethyl-3-ethoxy propionate.
 12. The substrate of claim11 wherein the solvent is ethyl-3-ethoxy propionate.
 13. The substrateof claim 10 wherein the novolak resin is resin B1.
 14. The substrate ofclaim 13 wherein the mixture of p-cresol and m-cresol used to preparenovolak resin B1 has a p-cresol and m-cresol weight ratio of from about60:40 to about 80:20.
 15. The substrate of claim 10 wherein the novolakresin is resin B2.
 16. The method of claim 10 wherein the positivephotoresist comprises from about 2 parts by weight of novolak resin perpart by weight of photosensitizer to about 6 parts by weight of novolakresin per part by weight of photosensitizer.
 17. The method of claim 16wherein the positive photoresist comprises about 4 parts by weight ofnovolak resin per part by weight of photosensitizer.